For reasons relating to the ever more crucial saving of fuel and in order to reduce pollutant emissions, it is common in the control of modern internal combustion engines for the supply of fuel to the cylinders to be reduced, or deactivated entirely, during overrun operation.
In general, overrun operation or trailing-throttle operation refers to a situation in which, with the throttle flap closed, the speed of the internal combustion engine lies above a certain predefined value, for example the idle speed. Overrun operation may however also be identified as a situation in which the internal combustion engine is at a speed higher than that which corresponds to the position of the throttle flap in the case of an Otto-cycle internal combustion engine or to the injected fuel quantity in the case of a diesel internal combustion engine. Instead of evaluating the throttle flap position or fuel quantity, it is also possible for consideration to be given to the signal of a pedal transducer of an accelerator pedal.
Since it is not desired for the internal combustion engine to perform work during overrun operation, it is possible in said operating state for the supply of fuel to be stopped, which is generally referred to as overrun cut-off or overrun deactivation.
Upon the transition from overrun operation to overrun cut-off, a step change in torque generally occurs owing to the interruption of the fuel supply, which step change in torque is under some circumstances perceptible in the vehicle as a jerk and can impair the running smoothness of the internal combustion engine and the driving comfort for the occupants of the vehicle that is driven by way of the internal combustion engine. The same problem arises if, after operation after overrun cut-off, it is the intention for normal driving operation of the vehicle to be resumed and for a desired torque to again be generated and output to the wheels of the vehicle by the internal combustion engine.
In this context, the expression “normal operation” is to be understood to mean fired operation of an internal combustion engine outside overrun cut-off, that is to say operation with an enabled fuel supply.
Various measures have already been proposed to reduce the undesired step change in torque that arises as a result of the deactivation or activation of the fuel injection during overrun operation.
For example, EP 0 614 003 B1 has described a method for controlling the fuel supply to an internal combustion engine with multiple cylinders and successive injections into the cylinders in a predefined sequence in the overrun operation range. Here, characteristic operating variables of the internal combustion engine are detected, and a start and an end of overrun operation are identified if said characteristic operating variables satisfy particular conditions. The fuel supply to the cylinders is, upon the start of overrun, reduced in accordance with a selectable function and, upon the end of overrun, is enabled again in accordance with a selectable function, wherein, after identification of overrun operation, the injections for individual cylinders are suppressed in accordance with a predefined sequence and, after identification of the end of overrun, the injections for individual cylinders are resumed in accordance with a further predefined sequence. The sequences are determined by way of a chronological sequence of individual suppression patterns stored in a memory of an electronic control apparatus. By means of such selective suppression of individual impulses of the individual cylinders, it is possible for the step change in torque upon the transitions to be considerably reduced.
DE 27 38 886 C3 describes a method for the electrical control of the operating behavior of an internal combustion engine with applied ignition in and after overrun operation, wherein the fuel supply can be shut off during said overrun operation and an ignition adjustment is performed. At the start, or proceeding from a predefined point in time after the start, of overrun operation, the ignition time is retarded in accordance with the selectable function, and, after the start of the overrun operation, the fuel supply is maintained for a definable time duration and, in response to a trigger signal, the ignition time is returned from a retarded position to the normal ignition time again in accordance with a selectable function in order to realize a smooth transition. The trigger signal is defined by the end of the overrun operation and/or the end of the shut-off of the fuel supply to the internal combustion engine by a signal from a transducer for the throttle flap opening angle and/or by a selectable engine speed. Subsequently, the retarding of the ignition time is reduced, and toward the end of the overrun operation, the fuel quantity supplied to the internal combustion engine is temporarily increased.
DE 103 34 401 B3 has disclosed a method and a device for controlling the transition between normal operation and operation with overrun cut-off in the case of an Otto-cycle engine operated with direct fuel injection, wherein, to avoid an inadmissible step change in torque that arises upon overrun cut-off as a result of deactivation of the fuel injection, the ignition angle is retarded. To further reduce the step change in torque, fuel is injected into a cylinder of the Otto-cycle engine in the form of multiple injections, wherein at least a partial amount of the fuel to be injected is deposited during the compression phase.